In the field of digital type fibre-optic telecommunications, in addition to the possibility of using techniques of a conventional type (usually referred to as “Non-Return-to-Zero”, or NRZ, in which essentially a 1 or 0 value is transmitted for the whole period corresponding to the speed of encoding adopted), the possibility exists of utilizing a transmission technology of the pulsed type, for example of the “soliton” or “soliton-like” type (usually referred to as “Return-to-Zero”, or RZ), in which essentially a sequence of pulses is transmitted, each lasting less than the period corresponding to the speed of encoding adopted, and modulated on the basis of the digital information to be transmitted.
One of the major problems encountered in the design of an RZ transmission system consists of the difficulty of generating pulses with an RZ profile which are suitable for this type of transmission, and, in particular, which are of sufficient time duration and are unaffected by “chirp”.
Among the techniques used for this purpose are known, for example, mode-locking fibre lasers and electroabsorption modulators, as well as other techniques.
A characteristic common to the aforesaid techniques consists of the fact that they are efficient for generating particularly short pulses suitable for transmission over dispersion-shifted or DS fibres, i.e. fibres with chromatic dispersion which approaches zero within the wavelength band employed for telecommunications, around 1550 nm, as for example defined by the ITU-T Recommendation G653 1993, and for time-division optical multiplexing.
In the case of transmission over so-called step-index fibres, or SI fibres, (as for example described in ITU-T Recommendations G650 1993 and G652 1993) and with dispersion compensation, it is useful to have available fairly long pulses (for example lasting from 20 to 60 ps for a transmission frequency of 10 Gbit/s) while it is observed that with shorter-lasting pulses in the SI fibre systems, with high dispersion, phenomena of dispersive wave generation are observed leading, ultimately, to an increase in the error rate of the transmission (BER).
Electroabsorption modulators, moreover, are intrinsically prone to generate pulses affected by “chirp”.
The term “chirp” is understood to mean a variation in the frequency of the signal during its amplitude modulation, so that there is a (central) frequency of the signal which is different at the start of the pulse from the (central) frequency of the signal at the end of the pulse.
The Patent WO 9616345 describes an apparatus which uses two amplitude modulators controlled by two phase-locked modulating voltages, one having double the frequency of the other, in which the larger is the speed of pulse repetition.
The article from the IEEE Journal of Selected Topics in Quantum Electronics, Vol. 2, No. 2, June 1996 (Veselka et al.) describes an apparatus which comprises several sinusoidally driven intensity modulators linked in series for forming pulses.
The Patent EP 622916 describes a soliton generator which comprises a phase modulator and an amplitude modulator, respectively driven at the frequency of pulse repetition and at a harmonically correlated lower frequency.
The Patent EP 718990 describes a device for converting a data stream of the NRZ type into an RZ stream, which employs a modulator with Mach-Zehnder interferometer or a directional coupler.
The U.S. Pat. No. 5,157,744 describes a soliton generator which comprises an amplitude modulator with Mach-Zehnder interferometer with a multiple series of distributed electrodes, driven at harmonically correlated frequencies. The patent states that the process of combining several high-frequency signals into a single signal involves large attenuations and requires amplification, and that the transmission and processing of the final signal, which is a composite of many high-frequency signals, is extremely difficult. Moreover, if the composite signal requires amplification, a very expensive amplifier is required that is able to amplify many very high frequencies uniformly. The invention of U.S. Pat. No. 5,157,744 is aimed at a soliton generator which avoids these problems.
D. Le Guen et al., in OFC 97 PD17(1–3), describe an experiment on a WDM soliton system with 10 channels at 20 Gbit/s, with compensation for chromatic dispersion and pre-chirping, in which a 1000 kilometer line of step-index fibre with 100 kilometer stretches was simulated by means of a 102 kilometer recirculation ring. The transmission uses electroabsorption modulators to modulate the emission from the laser sources so as to generate 20 ps pulses, subsequently coded by a lithium niobate modulator.
F. M. Knox et al., in ECOC 96, WeC3.2, 3.101–3.104, describe an experiment in 10 Gbit/s soliton transmission, with compensation for chromatic dispersion, over more than 2022 kilometers of step-index fibre; the experiment employed sech2 (t) pulses of around 20 ps at 2.5 GHz, generated by an active mode-locking erbium fibre ring and modulated with a pseudo-random bit sequence by a lithium niobate amplitude modulator and twice interleaved to give a data stream at 10 Gbit/s, and injected into a 33 kilometer recirculating ring with an appropriate module for compensating chromatic dispersion.
M. J. O'Mahony, in European Transactions on Telecommunications and Related Technologies, Vol. 4, No. 6, November–December 1993, pp. 629–640, presents and discusses the main design equations for a soliton system. A soliton transmission experiment over 3000 km of dispersion shifted fibre is also described. In the experiment, a train of soliton pulses with a FWHM duration of 35 ps is generated at a rate of 5 GHz by an InGaAsP electroabsorption modulator. A second electroabsorption modulator is used to impress 5 Gbit/s data on the pulse train. The technique of dispersion compensation in linear systems is disclosed as one of the possible alternatives to the technique of non-linear (soliton) transmission to overcome the limitation due to fibre dispersion for high bit rate operation over long distances (>1000 km).
EP 690534 discloses a semiconductor laser modulator used to simultaneously generate optical pulses and encode data, so as to output RZ soliton pulses suitable for transmission in long distance optical communications. One embodiment relates to a laser that is biased near threshold and is also directly encoded with digital data from a data source. A second embodiment relates to a laser modulator device. The laser is biased to output a CW laser beam which is then modulated by the modulator, controlled by an electrical pulse shaping circuit. A technique for adding further harmonics to the pulse shaping circuit is also disclosed, wherein a fundamental frequency sinusoidal signal is frequency doubled and the signal is combined with its second harmonic, to give a combined periodic analogue signal. A dual gate FET performs an AND operation of an input NRZ data stream and the combined periodic analogue signal, to produce a RZ-format signal corresponding to the NRZ data. The output of the FET is further amplified by an electronic amplifier before it is used to drive the modulator.
U.S. Pat. No. 5,504,609 discloses a remodulator for WDM optical communication systems. The remodulator includes an optoelectronic element for receiving an information bearing optical signal at a transmission wavelength and outputting a corresponding electrical signal. The remodulator further includes an optical carrier emitting element comprising a light source at a reception wavelength. It further includes an external modulator for directly imparting the information in the electrical signal on the optical carrier emitted by the light source.